1. Field
The present disclosure relates to extreme ultraviolet (“EUV”) light sources that provide EUV light from plasma created by converting a target material.
2. Background
Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers.
Methods for generating EUV light include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material having the required line-emitting element with a laser beam.
One LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with laser light pulses. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned to collect, direct (and in some arrangements, focus) the light at an intermediate location, e.g., a focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer.
In quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g., 10-200 kHz or more). There is also a need to deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position over relatively long periods of time. To ensure positional accuracy and repeatability, it is necessary to provide a high precision steering system that can release droplets from a range of positions to compensate for other systemic variations, for example, in laser targeting and timing. In this context, the term “steer” includes the concept of varying the position of the release point in at least two dimensions, i.e, with two angular degrees of freedom. It is also desirable to provide a steering system that is high bandwidth and that exhibits high stiffness with little or no hysteresis.
Design of a steering system meeting these criteria must also take into account that the droplet generator itself may be relatively massive, for example, on the order of 30 kg. The steering system also preferably operates over a relatively large range of angles, for example, with an actuation range of at least +1-2 degrees. Also, design considerations impose about a 1 micron requirement for position control of the droplets at the plasma location. This imposes a need for micro-radian level precision for the steering system.
With the above in mind, applicants disclose systems for steering a droplet generator.